1. Field of the Invention
The present invention relates to a torque detector for detecting torque without direct contact when an external force is applied to the rotating shaft of an automobile power-steering mechanism, etc.
2. Description of the Related Art
In an automobile power-steering mechanism, it is necessary to detect the amount of torque being applied to the steering wheel to determine the amount of power assistance required. The torque detector disclosed in Utility Model Laid Open No. 3-285130 is a known example of a conventional torque detector for this purpose. The construction of this device will be explained with reference to FIG. 5. In the figure, an upper shaft 2, which is attached to a steering wheel (not shown) and which is supported by a bearing 5a so as to be able to rotate freely, and a lower shaft 3, which is attached to the pinion gear of a steering mechanism (not shown) and which is supported by bearings 5b, 5c so as to be able to rotate freely, are disposed within a housing 1. A torsion bar 4, which is secured at its upper end to the upper shaft 2 by a pin 6 and is supported at its lower end by a bush 7 in the lower end of the upper shaft 2 so as to be able to rotate freely and at the same time is secured to the lower shaft 3, is disposed within the upper shaft 2 and lower shaft 3 so as to be disposed on the central axis of these shafts and is connected so as to be elastic in the direction of shaft twisting between the upper shaft 2 and lower shaft 3.
A first cylinder 10, which is composed of soft magnetic material and is secured to the upper shaft 2, has tooth portions 12 comprising a plurality of protrusions disposed equidistantly around it. A second cylinder 11, which is composed of soft magnetic material and is secured to the lower shaft 3, has tooth portions 13 comprising a plurality of protrusions disposed equidistantly around it. A coil 15, which detects changes in magnetic resistance between the first cylinder 10 and the second cylinder 11, is disposed radially outwards from the first cylinder 10 and the second cylinder 11.
Next, the operation will be explained. When torque is applied to the upper shaft by the steering wheel, torsional deformation occurs in the torsion bar 4, and angular shear occurs due to relative rotation between the first cylinder 10 and second cylinder 11, which are attached to the upper shaft 2 and the lower shaft 3, respectively, that is, relative angular displacement occurs between the upper shaft 2 and the lower shaft 3. When this happens, the surface area of the overlap which forms a magnetic circuit between the tooth portions 12 and 13 disposed on each of the cylinders changes and thus the magnetic resistance between the first cylinder 10 and the second cylinder 11 changes.
The torque can be determined by detecting this change in the magnetic resistance by means of the coil 15. However, in practice not only is torque being applied between the upper shaft 2 and lower shaft 3, but bending forces as well. The torsion bar 4, which connects the upper shaft 2 to the lower shaft 3, has little flexural rigidity and cannot withstand the bending forces on its own. For that reason, bending forces are prevented from being applied to the torsion bar by supporting it at both ends by means of the pin 6 and the bush 7.
Because the conventional torque detector is constructed in the manner described above, the torsion bar 4 requires a certain length and torsional rigidity corresponding to the relative angular displacement occurring between the upper shaft 2 and the lower shaft 3, but the longer the torsion bar is, the smaller its flexural rigidity becomes. Thus, in order to prevent the magnetic resistance between the first cylinder 10 and the second cylinder 11 from changing due to bending forces, there is a need to increase the rigidity of the upper shaft 2, the lower shaft 3, etc., and at the same time to use many supports, such as bushes and bearings. The problem is that increasing the number of supports leads to a loss of torque due to friction in the supports, and making the construction more complicated increases production costs.